The major objective of this proposal is to measure the translational mobility of various cell surface components in single, living cells using a laser excited fluorescene microscope. This will be accomplished by labelling the cell fluorescent molecules specific for the cell surface and either measuring the fluorescence recovery after photobleaching in small areas of the cell surface or by electronically observing spatial variations with time in the fluorescent image of the entire cell. Measurement of the mobility of plasma membrane components will be made when cultured cells are treated with various pharmacological agents, multivalent ligands to cell surface receptors, and growth factors and as a function of phase in the cell cycle and cell density and whether or not the cell has been neoplastically transformed. These measurements will assist in the development of a more refined understanding of the cell surface and the relationship of cell surface dynamics to the physiological situation of the cell. Accumulating evidence indicates that changes in the mobility of membrane components and membrane "fluidity", probably manifest in alterations in membrane transport, enzyme activity and cellular recognition phenomena, may play an important role in the development of certain diseases, particularly the neoplastic transformation.